1. Field of the Invention
The present invention relates to an imprint apparatus, and an article manufacturing method.
2. Description of the Related Art
As the demand for microfabrication of semiconductor devices increases, not only a conventional photolithography technology but also a microfabrication technology in which an uncured resin on a substrate is molded by a mold to thereby form a resin pattern on the substrate have been receiving attention. This technology is also referred to as an “imprint technology”, by which a fine structure with dimensions of a few nanometers can be formed on a substrate. One example of imprint technologies includes a photo-curing method. An imprint apparatus employing the photo-curing method first applies an ultraviolet curable resin (imprint resin, photocurable resin) to a shot area (imprint area) on a substrate (wafer). Next, the resin (uncured resin) is molded by a mold. After the ultraviolet curable resin is irradiated with ultraviolet light for curing, the cured resin is released from the mold, whereby a resin pattern is formed on the substrate.
In the imprint apparatus, an atmosphere in the apparatus is typically ambient air. Thus, ambient air may remain in the space between a mold and resin when the mold is pressed against the resin. Because of the entrapped air bubbles (residual gas) in the resin, defects may occur on the pattern to be transferred. Consequently, an accurate pattern may not be formed. In contrast, a method for waiting until residual gas is dissolved into, diffused from, or passed through a resin or a mold to thereby be extinguished may exist, however, a considerable amount of time is required for an imprinting step. Accordingly, U.S. Pat. No. 7,090,716 discloses an imprint apparatus that dissolves or diffuses the remaining a permeable gas into/from a resin or a mold to thereby decrease residual gas quickly by using a permeable gas as an imprint atmosphere. On the other hand, Japanese Patent No. 3700001 discloses an imprint apparatus in which a condensable gas to be condensed by pressure rise upon pressing a mold against a resin is used as an imprint atmosphere. Since the condensable gas is liquefied during remaining, and thus, its volume is reduced by up to a few hundredths of one percent in compared with its volume in the gaseous state, the effects of the residual gas on pattern formation may be suppressed.
However, in the imprint apparatuses disclosed in U.S. Pat. No. 7,090,716 and Japanese Patent No. 3700001, imprint atmosphere is totally purged by a permeable gas or a condensable gas, which is disadvantageous in terms of cost. Also, the imprint apparatus typically measures the position of a wafer stage or the like, which moves as appropriate, accurately using an interference displacement measuring unit. Since the measuring beam (light) is used, the measuring unit is very sensitive to changes in the composition of the medium (atmosphere) passing therethrough or changes in pressure and temperature. Thus, when the periphery of a resin on a substrate is purged by, for example, a gas having refractive index different from that of ambient air, the interference displacement may not accurately be measured if the gas enters the optical path of the measuring unit. Here, a permeable gas has smaller specific gravity than that of ambient air and has smaller refractive index than that of ambient air. Thus, if the permeable gas enters the optical path of the measuring beam, the refractive index of medium becomes small, and thus, interference displacement measurement may become unstable. On the other hand, a condensable gas has greater specific gravity than that of ambient air and has greater refractive index than that of ambient air. Thus, if the condensable gas enters the optical path of the measuring beam, the refractive index of medium becomes great, and thus, interference displacement measurement may also become unstable. If interference displacement measurement becomes unstable, the positioning of a wafer stage or the like becomes unstable. Consequently, the position of a pattern to be transferred deviates from the target position, and thus, accurate pattern formation may not be provided.